1. Field of the Invention
The invention relates to a power supply unit which includes an inverter which co-operates with a resonant circuit and can be controlled by means of a control unit so as to adjust the output power, notably for an X-ray generator.
2. Description of the Related Art
A power supply unit of this kind is known from DE 197 24 931 and includes an inverter which is connected to a direct voltage source and consists of a first branch and a second branch, each branch being provided with two semiconductor switches, the output of the inverter being connected to a resonant circuit that is formed by a capacitor and the stray inductance of a transformer. An X-ray tube is connected to the secondary winding of the transformer via a rectifier device. The power supply unit can be switched between a first mode of operation with a high short time power and a second mode of operation with a low continuous power, the power output being controlled by means of a pulse width modulated voltage which is generated by switching of the semiconductor switches and is present on the output of the inverter. In order to increase the continuous power in the second mode of operation, a third branch (auxiliary branch) is connected parallel to the two inverter branches; in this second mode of operation the auxiliary branch is activated instead of the first branch in order to take up an additional inductance and an additional capacitance in the primary circuit of the transformer. The power loss in the semiconductor switches of the inverter and in the transformer is thus reduced.
Even though power control by pulse width modulation of the voltage applied to the resonant circuit can be realized comparatively simply from a technical point of view, and even though practical no switch-off losses are induced in the case of low output powers, this method also has given drawbacks. This is because high switch-on and switch-off losses occur in the case of high output powers. Moreover, as the pulse width increases the output voltage also increases.
It is an object of the invention to provide a power supply unit of the kind set forth whose modes of operation are enhanced over a comparatively wide power range and notably in respect of switching losses and ripple of the output voltage.
This object is achieved by means of a power supply unit as claimed in claim 1 which is characterized in that the inverter is formed by a first switching unit and a second switching unit whereby a voltage of a first polarity can be applied to the resonant circuit in a first switching state whereas a voltage of a second polarity can be applied thereto in a second switching state, which switching states can be switched by the control unit in such a manner that in a first mode of operation for low output powers the output power can be adjusted by changing the duration of the switching states at an essentially constant switching frequency fs which is at least a predetermined factor lower than the resonance frequency fres of the resonant circuit, and that in a second mode of operation for high output powers the output power can be adjusted by changing the switching frequency in the range of the resonance frequency.
Thus, the basic idea of the invention is to optimize the properties of a power supply unit of the kind set forth by activation of different modes of operation in dependence on the output power.
Advantageous further embodiments of the invention are disclosed in the dependent claims.
The embodiment disclosed in claim 2 includes an inverter in the form of a half bridge (two semiconductor switches) whereas a full bridge circuit is provided according to claim 4 (four semiconductor switches).
When a third switching state is activated as a free-running state in conformity with claim 5, the peak values of the resonance current amplitudes are lowered, the losses are reduced further and the switching powers are halved. Moreover, the resonant circuit capacitance C is relieved by the unidirectional power flow.
In order to achieve optimum properties of the power supply unit, the factor whereby the switching frequency fs in the first mode of operation is smaller than the resonance frequency is preferably 0.5 in conformity with claim 6, whereas in conformity with claim 7 the switching frequency fs can be varied in the range 0.5*fresxe2x96xa1fs less than fres in the second mode of operation.
Finally, the embodiment disclosed in claim 8 involves a third mode of operation which can be effectively used notably for a medium range of the output power.
Further details, characteristics and advantages of the invention will become apparent from the following description of two preferred embodiments which is given with reference to the drawing. Therein: